When designing electrically steerable antennas (ESA) one strives for at least some of the following properties: low-weight, broad-band, dual polarisation, low losses, wide coverage, adequate packaging and a construction simple to manufacture as well as at low cost.
Balanced antenna elements such as the radiating portion of a notch element possesses excellent bandwidth properties, but are cumbersome to realise, e.g. to manufacture. One reason for this is that at least one transmission line per element needs to cross the ground plane, implying a feed through and possibly a contact.
The situation becomes even more complex when the possibility to switch between different polarisation states (e.g. linear and circular) is required.
A convenient way of feeding antenna elements above a ground plane is excitation by means of slots in the ground plane. This removes the need for a feed through or a contact
If wave-guide elements are to be tightly packaged and slot-fed with satisfactory results, see FIG. 1, they usually require electrically dense dielectrics in the wave-guide. However, such dielectrics tend to be far too heavy to be considered for use in large array antennas. As an alternative, protruding wave-guide ridges, with the ridge height gradually reduced towards the free-space end (in order to get good matching towards free-space) can be used. However there are doubts that high performance notch elements can be slot-fed because of seemingly disparate transverse field distributions.
Efficient antenna element design requires that the element volume be split at well-defined interfaces into several smaller volumes that can be optimised at a significantly lesser effort. However, a split interface in a protruding ridge/notch region of the antenna element implies boundary conditions not implemented in EM analysis software of today. On the other hand, a split interface in a wave-guide can be simulated with high accuracy.
Standard ridged wave-guide feeds do not easily fit into standard lowcost industrial manufacturing methods, while probe or stripline fed slots do, as doses a probe fed ridge.
A U.S. Pat. No. 6,577,207 from Jun. 10, 2003 discloses a dual-band electromagnetic coupler, which uses a ridged square wave-guide section to couple a square port of a mode converter to a common square port. The ridged square wave-guide section includes ridges and phase shifters which delay components of the high-band modes to produce a TE1,0 and a TE0,1 mode at the common port in both bands.
Another U.S. Pat. No. 6,552,691 from Apr. 22, 2003 discloses a broadband dual polarised micro-strip notch antenna. The phased array antennas includes two planar micro-strip notch elements that interlock and are perpendicular to each other having their phase centres coincident providing advantageous operational characteristics for forming wide bandwidth and wide scan angle.
Still another European patent EP0831550 discloses an antenna element consisting of a micro-strip section mounted at right angles to a support leaving a gap between the micro-strip section and the edge of the support. A notch starts from the free micro-strip edge. This has a wide section narrowing to a second narrower section. The notch dimensions provide a fixed phase centre in a narrow band of around 10% of desired centre frequency.
However these documents are considered to only constitute the state of the art and in any way not anticipating the present application.
Therefore there is still a wish to in a simple way obtain the properties desired to simultaneously achieve the requirements mentioned above and a solution for such a dually polarised wave-guide-notch antenna is here suggested by the present invention.